The present invention relates to a motor-driven fuel pump for pumping a fuel to an automotive engine, for example.
In a motor-driven fuel pump adapted to be disposed in a fuel tank for an automobile, for example, for pumping a fuel to an engine of the automobile, there has been employed a brushless motor suitable for high-speed rotation of a rotor under a high pressure of the fuel to be supplied (e.g., Japanese Utility Model Laid-open Publication No. 62-172272).
However, there is a problem of generation of a noise due to vibration caused by dynamic unbalance of the rotor. The dynamic unbalance of the rotor is caused by insufficient adjustment of dynamic balance of a rotor magnet mounted on a motor shaft since the rotor magnet has no portion to be cut out for the purpose of adjustment of the dynamic balance.
A sensor magnet for detecting a magnetic pole position is mounted to the rotor magnet with set screw after the rotor magnet adjusted in dynamic balance is mounted through a bearing to a motor housing. Accordingly, if the sensor magnet is eccentrically mounted to the rotor magnet to cause dynamic unbalance, the rotor magnet adjusted in dynamic balance generates dynamic unbalance because of the eccentricity of the sensor magnet.
FIG. 6 shows the relationship between a dynamic unbalance quantity of the rotor magnet and a noise in an automotive compartment in case of rotating the motor in the fuel pump at a speed of 4800 r.p.m. As apparent from FIG. 6, the noise increases linearly with an increase in the dynamic unbalance quantity.
In another structure of the brushless motor having a rotor assembly of the rotor magnet with the sensor magnet supported through a rotor bearing to the motor housing, the rotor bearing for supporting the outer circumference of the rotor assembly has a diameter larger (e.g., five times) than a motor shaft bearing for supporting the motor shaft. Accordingly, a peripheral speed of the bearing surface of the rotor assembly slidingly contacting the inner circumference of the rotor bearing becomes relatively high, causing an increase in frictional heat to be generated from the bearing surface. As a result, the rotor bearing is thermally expanded to cause a reduction in strength of the rotor bearing as well as damage of the motor shaft.
In such a motor-driven pump, the fuel discharged from the pump section into the motor housing is allowed to flow between the inner circumference of the motor housing and the outer circumference of a control circuit case fixedly mounted in the motor housing in opposed relationship to the sensor magnet. The fuel flowing at the outer circumference of the control circuit case functions to cool a control circuit in the control circuit case, e.g., a driving transistor in which a large current flows to cause large heat generation. However, the control circuit case cannot be sufficiently cooled by the fuel since it passes at the outer circumference of the control circuit case. In particular, when temperature of the fuel becomes high, fuel vapor is generated to stay on the outer surface of the control circuit case opposed to the sensor magnet, causing a reduction in cooling efficiency of the control circuit case.
Further, this type of motor-driven pump is vertically mounted in the fuel tank in such a manner that the motor shaft extends in a direction of a gravitational force. The rotor fixedly mounted on the motor shaft (which rotor corresponds to a rotor magnet in a brushless motor or a core in a brush D.C. motor, for example) is disposed in a vertically symmetrical relationship to the stator fixed to the motor housing. Accordingly, a magnetic attracting force between the rotor and the stator is applied in a direction perpendicular to the direction of the gravitational force, that is, in a horizontal direction. As a result, a weight of a rotary member including the rotor fixed to the motor shaft is applied to an upper end surface of a lower bearing for rotatably supporting the motor shaft at its lower end portion. Therefore, wearing of the upper end surface of the lower bearing during rotation of the rotor is accelerated to cause a reduction in durability of the lower bearing, resulting in a reduction in pump performance and an adverse effect to a fuel supply system due to the generation of wearing powder.